#! perl -slw
use strict;
use GD;

my $img = GD::Image->new( $ARGV[ 0 ] ) or die $!;

my $colors = $img->colorsTotal
    or die "$ARGV[ 0] is a truecolor image (no palette)";

for ( 0 .. $colors ) {
    printf "%5d: R:%2x G:%2x B:%2x\n", $_, $img->rgb( $_ );
}